The present invention relates to magnetoresistor arrays used for magnetic position sensors.
The use of magnetoresistors (MRs) and Hall devices as position sensors is well known in the art. For example, a magnetically biased differential MR sensor may be used to sense angular position of a rotating toothed wheel, as for example exemplified by U.S. Pat. Nos. 4,835,467, 5,731,702, and 5,754,042.
In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object moving relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the moving target wheel is adjacent to the MR than when a slot of the moving target wheel is adjacent to the MR.
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder heels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,731,702, and 5,754,042).
The shortcoming of MR devices is their temperature sensitivity. They have a negative temperature coefficient of resistance and their resistance can drop as much as 50% when heated to 180 degrees Celsius. Generally, this led to the use of MR devices in matched pairs for temperature compensation. Additionally, it is preferable to drive MR devices with current sources since, with the same available power supply, the output signal is nearly doubled in comparison with a constant voltage source.
To compensate for the MR resistance drop at higher temperatures, and thus, the magnitude decrease of the output signal resulting in decreased sensitivity of the MR device, it is also desirable to make the current of the current source automatically increase with the MR temperature increase. This is shown in U.S. Pat. No. 5,404,102 in which an active feedback circuit automatically adjusts the current of the current source in response to temperature variations of the MR device. It is also known that air gap variations between the MR device and ferromagnetic materials or objects will affect the resistance of MR devices with larger air gaps producing less resistance and decreased output signals.
Single element magnetic field sensors composed of, for example, an indium antimonide or indium arsenide epitaxial film strip supported on, for example, a monocrystalline elemental semiconductor substrate, are also known. The indium antimonide or indium arsenide film is, for example, either directly on the elemental semiconductor substrate or on an intermediate film that has a higher resistivity than that of silicon. A conductive contact is located at either end of the epitaxial film, and a plurality of metallic (gold) shorting bars are on, and regularly spaced along, the epitaxial film. Examples thereof are exemplified by U.S. Pat. Nos. 5,153,557, 5,184,106 and 5,491,461.
Most noncontacting magnetic angle position sensors use a Hall sensor and a rotating magnetic field. Since the Hall sensor output signal is proportional to the normal component of the magnetic field, its output is a sinusoidal function of the angle of rotation. Only within a relatively small angular range is the output proportional to the angle of rotation. Depending on the required accuracy, this range may be as small as xc2x130 degrees with a xc2x11.3% full scale error and, practically, never greater than xc2x150 degrees with almost a xc2x110% full scale error. Another approach relies on varying the air gap between a Hall sensor and a magnetic target. This allows a greater angular range. However, it is an inherently error prone method due to the high degree of non linearity in the relation between the magnetic field strength and the air gap.
Compound semiconductor MRs, such as those manufactured from lnSb, InAs, etc., are simply two-terminal resistors with a high magnetic sensitivity and thus, are very suitable for the construction of single die MR array geometries suitable for use as large range angular position sensors (in most cases one terminal of all the MR elements can be common).
Ultimately, such MR arrays could be integrated on the same die with appropriate processing circuitry. For example, if the MR array was fabricated on a Si substrate then the processing circuitry would be also Si based. For higher operating temperatures, silicon-on-insulator (SOI) could be used. A potentially lower cost alternative to the SOl approach would be to take advantage of the fact that MRs are currently fabricated on GaAs, a high temperature semiconductor, and thus, to fabricate the integrated processing circuitry from GaAs (or related lnP) using HBT (Heterojunction Bipolar Transistor) or HEMT (High Electron Mobility Transistor) structures. This technology is now easily available and inexpensive through the explosive growth of the cellular phone industry.
Accordingly, what remains needed is a compact and inexpensive die having at least one array of magnetic sensing elements and configured so as to produce a variety of array geometries suitable for specialized angular sensing schemes capable of self compensation over wide ranges of temperature and air gaps, wherein an array is defined as having three or more MR elements.
The present invention is a compact and inexpensive single die having at least one MR array composed of a plurality of MR elements, wherein each MR element is composed of a number of serially connected MR segments. The MR elements are arranged and configured so as to produce a variety of MR array geometries suitable for specialized angular sensing schemes.
The present invention is a noncontacting large angular range (approaching 180 degrees) angular magnetoresistor position sensor array incorporated on a die capable of self compensation over wide temperature ranges and air gaps.
According to a first aspect of the present invention, an MR array is formed of a plurality of MR elements, wherein each MR element is composed of a plurality of uniformly arranged, serially connected MR segments. The arrangement is such as to provide an MR array suitable for angular sensing schemes wherein angular measurement redundancy is incorporated therein.
According to a second aspect of the present invention, an MR array is formed of a plurality of MR elements, wherein each MR element is composed of a plurality of uniformly arranged, serially connected MR segments. The arrangement is such as to provide an MR array suitable for angular sensing schemes wherein angular measurement redundancy and reference redundancy are incorporated therein.
According to a preferred method of fabrication, an indium antimonide epitaxial film is formed, then masked and etched to thereby provide epitaxial mesas characterizing the MR elements. Shorting bars, preferably of gold, are thereupon deposited, wherein the epitaxial mesa not covered by the shorting bars provides the MR segments. The techniques for fabricating epitaxial mesas with shorting bars are elaborated in U.S. Pat. No 5,153,557, issued Oct. 6, 1992, U.S. Pat. No 5,184,106, issued Feb. 2, 1993 and U.S. Pat. No. 5,491,461, issued Feb. 13, 1996, each of which being hereby incorporated herein by reference.
Accordingly, it is an object of the present invention to provide an MR die comprising at least one MR array according to the first and second aspects of the present invention which is capable of detecting angular movement of a ferromagnetic or magnetic target in relation to the MR array.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.